1. Field of the Invention
This invention relates generally to partial discharge technology and specifically to on-line sensing of partial discharge utilizing insulator stress shields as capacitive sensing devices.
2. Description of Prior Art
Previously, the on-line monitoring of high-voltage insulation of electrical equipment was performed on the equipment under operation, i.e. in the actual operating condition. Equipment de-energization is required only for the initial sensor installation. As increasingly reliable and cost- and labor-effective, this technology is now widespread in numerous applications.
Attention is called to the following Publications:
xe2x80x9cMethods and Tools for High-Voltage Equipment Diagnosticsxe2x80x9d, Energoatomizdat Publishing House, Moscow, by P. Svy 1992. xe2x80x9cExperience in the Application of the On-Line Monitoring System Using Power Frequency and Partial Discharges to High Voltage Transformer and Bushing Insulationxe2x80x9d, by Z. Berler, L. Letitskaya and P. Svy, EPRI Substation Equipment Diagnostic Conference VI, Feb. 16-18, 1998, New Orleans, La.
These publications teach that on-line monitoring of sensor frequency and radio frequencies for predicting and prevention of high voltage equipment failures in service.
Bushings of power transformers, shunt reactors or circuit breakers and current transformers, comprised of oil-impregnated paper insulation, such as used also in cables or capacitors, are equipped with so called capacitance or potential taps. A capacitance tap is connected to a metal shield inserted inside the insulation. The insulation has certain capacitance with reference to the high voltage current-carrying conductor. Both the capacitance value and the power factor of the insulation depend upon the insulation condition and could be quantified at the tap output with the equipment on-line. Furthermore, the electrical impulses that accompany partial discharges inside the insulation are also coupled to the output of the capacitance tap and can be detected using circuits of a suitable design.
The capacitance taps were originally designed only for relatively rare off-line insulation tests using suitable test source at power frequency. During equipment operation they remained grounded. It was recognized soon that these taps lend themselves as excellent means of on-line monitoring of the insulation. The use of the capacitance tap for an on-line monitor requires a sensing device to be inserted between the live tap contact and the ground. The aforementioned publications teach such an arrangement.
The sensor designed for the power frequency measurement produces a signal proportional to the capacitive current through the bushing insulation. The sensor designed for partial discharges senses the radio-frequency impulses and produces a signal of magnitude proportional to the dissipated electrical charges.
The repetition rate of such discharges can be determined by a measuring device.
Such sensors are described in U.S. Pat. No. 5,471,144 xe2x80x9cSystem for Monitoring the Insulation Quality of Step Graded Insulated High Voltage Apparatusxe2x80x9d issued Nov. 29, 1995; U.S. Pat. No. 5,574,378 xe2x80x9cInsulation Monitoring System for Insulated High Voltage Apparatusxe2x80x9d issued Nov. 12, 1996; U.S. Pat. No. 5,640,154 xe2x80x9cInsulation Monitoring System for Insulated High Voltage Apparatusxe2x80x9d issued Jun. 17, 1997; and U.S. Pat. No. 5,652,521 xe2x80x9cInsulation Monitoring System for Insulated High Voltage Apparatusxe2x80x9d issued Jul. 29, 1997. They consist of a current transformer with a primary winding created by the capacitance tap grounding conductor, and a secondary toroidal winding consisting of several or many turns. This current transformer can be coreless (so-called Rogovsky coil), as suggested in the above mentioned patents for power frequency measurements, or with a ferrite core, as recommended in the Svy Reference for the radio-frequency impulse measurements. The advantage of the current transformer-based sensor is its simplicity. A current transformer with its secondary winding loaded with a small resistance has small input impedance, so there is usually no need for a special tap overvoltage protection.
Monitoring of radio frequency (partial discharge) impulses imposes different requirements on sensor design, as opposed to monitoring of signals at power frequency. For partial discharge monitoring it is desirable to detect a frequency band generally between 0.5 and 20 MHz with high sensitivity. Ferrite radio-frequency transformers are appropriate for this task as they are capable of accurately transmitting short and steep pulses, but they block power frequency signal. A coreless current transformer can be employed, which, on the other hand, is practically insensitive to weak partial discharge pulses. To meet both requirements, two separate transformers, one of each type, are necessary.
A coreless Rogovsky coil has a low sensitivity even at the power frequency signals. The measured quantity, a power frequency voltage drop across the resistor shunt, is directly proportional to the capacitive current through the bushing insulation. The magnitude of the power frequency signal can be conveniently controlled by the resistance chosen for the shunt. The disadvantage of such arrangement is that the tap capacitance, between the high voltage line and the output of the capacitance tap, in series with the resistance, represents a frequency dependent voltage divider. As a result, switching and lightning transients can cause overvoltages at the output of the tap due to their very high frequencies. These transients have the potential of destroying not only the measuring circuit, but also the insulation of the tap output. To limit the transients, a surge arrestor is added in parallel to the resistor shunt, as shown in the Svy Reference.
A further improvement of the sensor consisted of replacing the resistor shunt with another capacitor, see U.S. Pat. No. 4,757,263 xe2x80x9cInsulation Power Factor Alarm Monitorxe2x80x9d issued Jul. 12, 1988; U.S. Pat. No. 5,903,158 xe2x80x9cMonitoring of Internal Partial Discharges in a Power Transformerxe2x80x9d issued May 11, 1999; and U.S. Pat. NO. 6,028,430 xe2x80x9cMethod for Monitoring a Capacitor Bushing, and Monitoring Systemxe2x80x9d issued Feb. 22, 2000. This arrangement features a capacitor divider ratio that is essentially independent of frequency, thus minimizing the exposure of the tap and the low voltage circuits to destructive switching and lightning impulses. A surge arrester is kept in place as a xe2x80x9csecond line of defensexe2x80x9d for rare cases of extremely severe overvoltages.
All of the sensor designs described above are mutually exclusive in that they can satisfy only one application at a time; a power frequency signal detection or a partial discharge detection, but not both. With only one capacitance tap available per bushing, this represented a serious disadvantage as the replacement of a bushing sensor requires outage.
A Publication entitled xe2x80x9cOn-Line Monitoring of Power Transformer-Trends, New Developments and First Experiencesxe2x80x9d by T. Leibfried, W. Knorr K. Viereck, CIGRE, 1998, #12-211, teaches a sensor that can contain both circuits. The sensor relies on the capacitor shunt connected to the tap output and the radio frequency current transformer the primary winding of which is connected in series with the capacitor shunt, either on its grounded side or on its xe2x80x9clivexe2x80x9d side. Two separate coaxial cables carry power frequency and radio frequency signal signals respectively.
In the past, in monitoring partial discharges special bushing insulators were required to be placed in the high voltage line. These insulators had capacitor plates disposed therein for acting as sensors for sensing partial discharge.
The disadvantage of this type of sensor is the necessity of providing a separate sensing insulator rather than using preexisting insulator devices which could be adapted to act as a partial discharge sensor.
It would be advantageous if a system could be found that permitted the utilization of existing technology for adaptation to be utilized as a partial discharge sensor capacitive element.
In accordance with the invention, a partial discharge sensor system for an electrical system is taught. The sensor includes a conductor at a given voltage potential. There is an electrical insulator disposed around the conductor. A potential stress-reducing shield is concentrically disposed around the conductor. The stress shield, conductor and electrical Insulator cooperate as a capacitor which conducts partial discharge electrical current therethrough. A sensor current device is connected electrically to the tap to conduct the partial discharge electrical current therethrough from the capacitor.